Labour MPs Michael Meacher and John McDonnell - who had been planning separate leadership challenges - last week announced they were uniting in their bid to target Mr Brown.

They agreed one of them would withdraw, depending on the level of backing they had when Mr Blair steps down.

Mr Blair said earlier this week that "in all probability" Mr Brown would succeed him as prime minister "in the next few weeks".

If there is only a challenge from a left-wing candidate, Mr Blair is expected to come out in support of Mr Brown once the contest to replace him gets under way - although he has yet to formally back him.

Mr Blair is to announce his resignation next week, after which he will stay as PM for about seven weeks until Labour chooses his successor.